The present invention relates to integrated circuit devices, and in particular high voltage semiconductor switching devices such as high voltage transistors, power MOSFETs, power IGBTs, thyristors, MCTs, and the like (hereinafter called power devices).
Conventional power devices are fabricated by conventional semiconductor processing techniques on a single crystalline semiconductor substrate such as a silicon wafer. Conventional semiconductor processing techniques include doping and implanting, lithography, diffusion, chemical vapor deposition (CVD), wet and dry etching, sputtering, epitaxy, oxidizing, among others. A complex sequence of these processing techniques is often required to produce a high voltage device having a breakdown voltage within the 30 to 1200 volt range.
A limitation with the conventional power device is its shallow junction region. The shallow junction region often creates low junction curvature and reduces the breakdown voltage of the device. This lower breakdown voltage is often an undesirable result for high voltage applications.
Industry has proposed or even attempted to overcome such limitation with use of a guard ring formed adjacent to the main junction of the power device. The guard ring typically provides a junction termination technique for the conventional power device. A conventional guard ring is often formed by selectively placing certain dopants around the periphery of the main junction, typically in a "race track" or "ring" type pattern. The dopants often include impurities of the same impurity type as the main junction. Ideally, the guard ring keeps the main junction in its place.
However, as industry demands for power devices with even higher breakdown voltages and even smaller device features, the presence of contamination on certain portions of a conventional guard ring structure detrimentally effects an electric field therein, thereby degrading the breakdown voltage of the device. Accordingly, the presence of contamination often creates a resulting power device that is unstable, unreliable, or the like.
Another technique often used to preserve the breakdown voltage of the device is to form a field plate located between certain guard rings for the purpose of reducing electric fields thereby. The field plate is formed overlying an oxide layer, also located overlying regions between the guard rings. Ideally, lower electric fields at such location should tend to increase the breakdown voltage of the device. However, a limitation with the field plate structure often occurs with power devices having higher breakdown voltages.
For example, power devices with even higher breakdown voltages produce an even higher electric field underneath portions of the oxide layer. The higher electric field generally promotes certain hot electron effects such as electrons being injected and trapped into portions of the oxide layer, and the like. As charge builds up in the oxide layer from the trapped electrons, the conventional device often experiences detrimental effects such as current leakage, voltage instability, unreliability, and the like.
From the above, it is seen that a method and structure for providing a device with a high breakdown voltage that is easy to manufacture, reliable, and cost effective is often desired.